1. Field of the Invention
This invention relates to vibration dampers for shrouded turbine blades, and especially to dampers to inhibit both untwist motion imposed on the blades and circumferential motion between shrouded blade groups.
2. Description of the Prior Art
As is known to those skilled in the art of steam power generation, a steam turbine comprises a rotating member having mounted thereon a plurality of annular arrays of rotating blades alternating between arrays of stationary nozzles mounted within a casing surrounding the rotor. High pressure, high temperature steam is confined and guided within the casing and into the rotating blades by the nozzles to convert the energy of the steam to rotational mechanical energy.
The rotating blades comprise a root portion securely fastened to the rotor and a radially extending portion having an air foil cross section. Such blades are designed and manufactured so as to change shape and angle as the blade extends radially outward, so as to accommodate changes in linear speed of the blade as one advances from its root towards its tip. It has been known in the art that these twisted blades tend to untwist as the rotor spins at high speeds due to the imposition of centrifugal force thereon. As shown in U.S. Pat. No. 2,510,734, such untwisting action can be utilized to provide vibration damping.
As shown in the last mentioned U.S. patent and in the patent to Trumpler, U.S. Pat. No. 3,795,462, issued to the assignee of the present invention, frictional interfaces are provided between groupings of blades so that the imposition of the untwist force exerts frictional forces along the last mentioned interfaces which inhibit vibration forces imposed on the blades.